Coal is one of the main energy resources for human kind. As the petroleum reserve is getting less and less, the proportion of coal in the constitution of energy will be getting larger and larger. Unfortunately, much of the coal found in the world contains sulphur in varying quantities. When such sulphur-containing coal is burned, sulphur dioxide is produced and emitted into the atmosphere, which severely pollutes the atmosphere of the globe. Therefore it is very important to device a process for "clean burning" of sulphur-containing coal to reduce the pollution of the atmosphere by smoke when sulphur-containing coal is burned.
U.S. Pat. No. 4,226,601 disclosed a "Process for reducing sulphur contaminant emissions from burning coal or lignite that contains sulphur". In the disclosed process, the coal or lignite that contains sulphur is first pulverized then directly mixed with a finely divided inorganic material for burning. The inorganic material can be at least one of the oxides, hydroxides or carbonates of sodium, potassium, calcium or barium or it can be dolomite. Another way is to form the mixture of coal or lignite and the inorganic material into briquettes or pellets in order to reduce sulphur dioxide contained in the smoke, the mixture could be conveniently shipped, stored and used in conventional combustion equipment.
U.S. Pat. No. 4,322,218 discloses a process in the title of "SO.sub.2 capture-coal combustion". In the disclosed process, a coal, a calcium-containing material and a additive are blended homogeneously prior to their introduction into the burner; or coal, calcium containing meterial and other additive are put simultaneously into the burner.
The disadvantages of the above disclosed processes are: only a sulphur trapping agent composition of calcium series is simply blended with sulphur-containing coal for reducing SO.sub.2 emission from burning sulphur-containing coal. As the specific gravity of the calcium-containing sulphur trapping agent composition is much larger than that of coal or lignite, when a certain amount of said composition is added to the coal, the ratios of the volume and total surface area of said composition will be much smaller than that of coal or lignite. When sulphur-containing coal or lignite is burned, the sulphur dioxide formed has less opportunity to contact the calcium-containing sulphur trapping agent. As a result, the conversion rate of SO.sub.2 to CaSO.sub.3 and/or CaSO.sub.4 is low, SO.sub.2 content in smoke is still high. Therefore, the above-disclosed processes can not reduce the smoke pollution of atmosphere effectively.
To overcome the demerits of the prior arts, the object of this invention is to provide a process for reducing the SO.sub.2 emission from burning sulphur-containing coal which includes anthracite coal, bituminous coal, subbituminous coal and lignite effectively. Another object of this invention is to increase the contacting opportunity between SO.sub.2 and calcium-containing sulphur trapping agent composition in order to raise the coefficient of utilization of the composition itself and reduce the SO.sub.2 content in smoke effectively when a small amount of the sulphur trapping agent composition is used.
The sulphur trapping agent composition in the present invention comprises conventional calcium-containing compounds such as calcium oxide, calcium hydroxide or calcium carbonate, and at least one compound selected from the group consisting of the inorganic compounds of potassium, sodium, barium, manganese, tin and ferro. In said composition, a calcium-containing compound is more than 90 percent of the total weight, and the other material is less than 10 percent of the total weight. It is well-known by the person skilled in this field that the inorganic compounds of potassium, sodium, barium, manganese, tin and ferro mainly act as a catalyst in sulphur trapping reactions.
The advantages of the present invention are as follows:
1) Further reducing the SO.sub.2 content in smoke from burning sulphur-containing coal:
By applying the technology of the present invention, the SO.sub.2 content in the smoke is about 20% to about 40% lower than that by adopting the conventional technology.
2) Reducing the consumption of inorganic sulphur trapping agent:
The atomic ratio of calcium in the composition to sulphur in coal present in is from about 1.0 to about 2.0 in the present invention in comparison with that the atomic ratio of calcium to sulphur is larger than 2.0 in the prior arts in order to obtain the commercial effects of the sulphur trapping according to the present invention.